Satellite navigation systems (SNS's) allow electronic receivers to determine their own location (longitude, latitude, and altitude) to within a few meters, using time signals transmitted along a radio line of sight from satellites. The signals also allow the receivers to calculate the current local time to high precision, which allows time synchronization.
A satellite navigation system with global coverage may be termed a global navigation satellite system or GNSS. As of 2013, only the United States NAVSTAR Global Positioning System (GPS) and the Russian GLONASS are operational GNSSs. China is in the process of expanding its regional Beidou navigation system, and the European Union's Galileo system is in progress. Other countries are in the process of developing global or regional navigation systems.
SNS's have three parts: the space segment, the user segment, and the control segment. The space segment consists of a number of satellites, each in its own orbit above the Earth. The user segment consists of receivers, which can be very small and can be either stationary or mobile. The control segment consists of ground stations that measure the satellite orbits and maintain the satellite based time standards.
In operation, SNS satellites each broadcast a signal that contains the orbital parameters of the satellite and the time on board that satellite. The satellite uses a highly stable atomic clock as a master reference for its frequency generation and time keeping. An SNS receiver subtracts the time of broadcast encoded in the transmission from the time of reception, as measured by its internal clock. This difference, when multiplied by the speed of light, provides a range measurement between the satellite and the receiver. The SNS receiver's location and its clock offset from the time reference of the satellite is unknown, but can be calculated from the simultaneous range measurement to at least four satellites. The receiver also receives the orbital ephemeris for the satellites, which allow it to calculate the position of the satellite when the signal was transmitted. Once the positions of the satellites at the time of transmission are known, the location of the receiver and the receiver's clock offset is then calculated.